Variable-frequency speed control of high-power motors is an important means to improve energy efficiency of motors. With the continuous progress of power electronic devices, high-power motor variable-frequency speed control systems are widely applied. Variable-frequency drive technical solutions which are commonly used at present are mainly centralized on adding a frequency converter to a common motor, do not consider a variable-frequency motor and a variable-frequency speed control system as one system architecture, and have some limitations. For example, the most commonly used drive solution is that a high-voltage three-phase motor is added with cascade high-voltage frequency converters, weaknesses thereof lie in that levels of a plurality of independent H bridges of each phase are different, insulation treatment must be performed and it is difficult to realize liquid heat dissipation and cooling; although a three-level variable-frequency speed control solution can solve the problem of common ground of power devices, due to the limitations of withstand voltage and current of power devices, high-voltage and large-current output cannot be realized; although a multi-level variable-frequency speed control solution can break through certain voltage limitations, voltage sharing technique and hardware structure are very complex; and a two-level three-phase variable-frequency speed control solution is restricted by voltage and current. On a motor side, due to the increase of power supply voltage, the requirements on insulating systems and manufacturing processes of high-voltage motors are also greatly increased, the insulation life of motors is reduced, the manufacturing process cost is increased, the heat dissipation conditions of motors become poor and the power density is decreased.
In consideration of these factors, the present invention is particularly provided to solve the above-mentioned problems.